The present invention relates to an arc tube employed as a light source body in a discharge bulb. Recently, discharge bulbs have been extensively employed as light sources for a headlamp of a motor vehicle.
A discharge bulb is constructed generally as shown in FIG. 10. An arc tube 4 is supported by a pair of metal lead supports 2 and 3, which are embedded in an insulating base 1. To form the arc tube 4, a quartz glass tube is pinch-sealed at both ends, thus providing two pinch-sealed portions 4b, and a discharge section, namely, a closed glass bulb 4a between the pinch-sealed portions 4b. A pair of electrode assemblies 5 and 5, each of which is formed by a tungsten electrode bar 5a, a molybdenum foil 5b and a molybdenum lead wire 5c, are sealingly held in respective ones of the pinch-sealed portions 4b in such a manner that the electrode bars 5a protrude from the pinch-sealed portions into the closed glass bulb 4a, thus forming the aforementioned discharge section, and the lead wires 5c protrude outside from the pinch-sealed portions 4b and are welded to the lead supports 2 and 3, respectively.
To form the arc tube, a pair of electrode assemblies 5 as shown in FIG. 11 are prepared. Each of the electrode assemblies 5 is formed by connecting an electrode bar 5a and a molybdenum foil 5b as a single unit. Next, as shown in FIG. 12(a), while a forming gas is supplied into a quartz glass tube, a glass bulb 4a is formed with a metal mold. Thereafter, as shown in FIG. 12(b), an exhaust tube 6 is connected to the glass bulb 4a, the two electrode assemblies 5 are inserted into the quartz glass tube from both ends, and both end portions of the quartz glass tube where the electrode assemblies are set are heated and then pinch-sealed as shown in FIG. 12(c). Under this condition, as shown in FIG. 12(d), mercury and a metal iodide or the like is supplied into the glass bulb 4a through the exhaust tube 6. Thereafter, while xenon gas is supplied into the glass tube 4a through the exhaust tube 6, the exhaust tube 6 is broken off, as shown in FIG. 12(e). Subsequently, as shown in FIG. 12(f), the glass bulb 4a is cooled so that the xenon gas is held a liquid state, and the exhaust tube 6 is cut off again, this time closer to the bulb 4a, with a carbon dioxide laser.
In the above-described manufacture of the arc tube, the exhaust tube 6 is connected to the glass tube to form the T-shaped glass tube, and after the light emitting material is supplied to the glass tube through the exhaust tube 6, the latter 6 is broken off. Therefore, the closed glass bulb 4a includes a broken-off portion 4a (FIG. 10) which is formed when the exhaust tube 6 is broken off.
In order to ignite the arc tube quickly, it is necessary to orient it in such a manner that the broken-off portion 4a.sub.1 is at the top. This will be described in more detail.
In order to accelerate the gasification of the light emitting material (mercury or metal iodide), it is essential that the temperature of the entire closed glass bulb 4a be increased rapidly. On the other hand, of the closed glass bulb, the broken-off portion 4a.sub.1 is generally lowest in temperature since it is has a sharp protrusion. Therefore, the arc tube should be so oriented in such a manner that the broken-off portion 4a.sub.1 is the uppermost part of the closed glass bulb, which part is readily increased in temperature.
On the other hand, with respect to the distribution of light, light emitted from the upper portion of the glass bulb 4a is most effective. However, the broken-off portion 4a.sub.1 when at the top can scatter light, producing glare.
In addition, gravity acts on the arc formed between the electrodes, curving it downward. The total amount of curvature of the arc is proportional to the distance between the arc and the inner surface of the glass bulb. Therefore, the broken-off portion 4a.sub.1 increases the curvature of arc. In addition, the curvature of the arc depends on the position of the broken-off portion 4a.sub.1. Accordingly, in assembling the discharge bulb, it is essential that the glass bulb be positioned so that the broken-off portion 4a.sub.1 is the uppermost part thereof. This requirement is a factor which, in assembling the discharge bulb, lowers the work efficiency.